1. Field of the Invention
The present invention relates to an ink composition and an inkjet recording method and in particular to an ink composition and an inkjet recording method which are used preferably for inkjet recording by curing with high sensitivity, by irradiation with active energy rays.
2. Related Art
An image recording method of forming an image on a recording medium such as paper based on an image data signal includes an electrophotographic method, sublimation and fusion type thermal transfer methods and an inkjet method. Particularly the inkjet system involves jetting an ink onto necessary image regions with an inexpensive apparatus thereby forming the image directly on a recording medium, thus enabling effective use of ink and reduction in running costs. Moreover, the inkjet system is excellent as an image recording system because of fewer noises.
The inkjet system allows printing not only on plain paper but also on non-water-absorptive recording media such as plastic sheet and metal plate, but represents important issues for higher speed in printing and for higher image properties and suffers from drying of droplets after printing and the time required for curing, thus having a property of being significantly influenced by productivity of prints and the sharpness of printed images.
As one inkjet system, there is a recording system of using a curable inkjet recording ink by irradiation with radiations. This system involves irradiation with radiations immediately after ink jetting or within a given time period after jetting, to cure ink droplets thereby improving productivity in printing to form sharp images.
By achieving higher sensitivity in an inkjet recording ink that can be cured by exposure to radiations such as ultraviolet rays, the ink produces a large number of benefits such as high curability conferred toward radiations, improvement in productivity of inkjet recording, reduction in power consumption, longer lifetime due to decrease in the load on a radiation generator, and prevention of formation of low-molecular-weight material volatilizing from insufficient curing. The achievement in higher sensitivity leads in particular to improvement in the strength of images formed from the inkjet recording ink.
Preferable properties of images formed from the ink include adhesiveness to a substrate and flexibility of images. Particularly when images are formed on the surface of an uneven substrate or on the surface of a flexible substrate such as a resin film and further when a print molding (for example a resin bottle) having a molding formed in a state provided thereon with a print is formed, the flexibility of images becomes an important factor and is made hardly compatible with curability. That is, when the ratio of monofunctional monomers is increased for improving flexibility, there may be causes wherein curability is decreased or the image surface tackiness is caused due to blurring of uncured low-molecular-weight components. Generation of surface tackiness results in the blocking of prints formed from an ink composition, deterioration in workability, and deterioration in images formed, so there has been demand for suppression thereof.
For improving adhesiveness, flexibility and curability, techniques of using low-viscosity urethane acrylates in an inkjet ink composition have been proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-167537). However, this ink composition hardly maintains sufficient curability when the content of monofunctional monomers is increased to form a more flexible ink film.
Curable ink compositions, particularly radical-polymerizable ink compositions, are liable to polymerization inhibition with oxygen adjacent to the surfaces of images formed and are considered subject to surface tackiness attributable to reduction in the curing property of the surfaces. Accordingly, techniques wherein surface-oriented functional groups are introduced into a polymerization initiator and distributed unevenly on the surface of the polymerization initiator have been proposed (see, for example, Japanese Patent Application National Publication (Laid-Open) No. 2004-506639).
However, the compound having an ability to initiate polymerization as described herein has failed to sufficiently attain an effect of suppressing the blocking caused by surface tackiness due to a low glass transition temperature of the compound.